Method of galvanizing employing rolls of an air hardening die steel

ABSTRACT

METHOD OF GALVANIZING FERROUS STAND UTILIZING ROLLS OF AN AIR HARDENING DIE STEEL. IN GALVANIZING APPARATUS, SINKER, STABILIZER AND EXIT ROLLS SUBJECTED TO MOLTEN ZINC CONSISTING OF AN AIR HARDENING DIE SHEET WHICH IS WETTABLE BY MOLTEN ZINC AND SUBSTANTIALLY RESISTANT TO FORMATION OF IRON-ZINC ALLOY.

Jan. 5, 1971 P. E. SCHNEDLER 3,553,004 METHOD OF GALVANIZING EMPLOYINGROLLS OF AN AIR HARDENING DIE STEEL Filed Feb. 10,- 1969 Fig. 2 3

mvsmoiz PAUL E. jam/Emma,

' ATTORNEYS United States Patent Int. (:1. C23c 1/02 US. Cl. 117-114 3Claims ABSTRACT OF THE DISCLOSURE Method of galvanizing ferrous strandutilizing rolls of an air hardening die steel. In galvanizing apparatus,sinker, stabilizer and exit rolls subjected to molten zinc consisting ofan air hardening die steel which is wettable by molten zinc andsubstantially resistant to formation of iron-zinc alloy.

CROSS REFERENCE TO RELATED APPLICATION This is a continuation-in-part ofapplication Ser. No. 481,865 filed Aug. 23, 1965, now abandoned, in thename of Paul E. Schnedler and entitled Coating Method.

BACKGROUND OF THE INVENTION This invention relates to the continuouscoating of ferrous strand-like material with molten zinc and moreparticularly to a new alloy steel roll for such a coating operation. Itwill be understood that the term strand as used hereinafter in thisapplication will refer generally to ferrous materials adapted to becoated in a continuous galvanizing operation As such, the term includesstrip, sheet, wire, tubing, or other forms of ferrous material.

According to the prior art, the ferrous strand has its surface preparedin various ways to insure receptivity of the molten coating metal. Afterthis preliminary preparation the strand is passed into a bath of moltenzinc and under a pot or sinker roll, then, optionally past one or morestabilizer rolls. As the strand is withdrawn from the zinc bath, exitrolls or coating rolls are often used for the purpose of smoothing outthe coating and to control the coating weight within a desirable range.

Exit rolls are normally made of steel having a carbon content in therange of 0.01 to 0.30 and are normally used in the annealed condition.Such a material will alloy rapidly with the molten zinc of the coatingbath forming a thick, easily removable iron-zinc alloy which has greatervolume than the original iron. In the case of exit rolls, a skillfuloperator uses the known characteristics of this iron-zinc alloy in orderto maintain the shape or, if necessary, to change the shape of the exitrolls in order to suit his needs. That is, in order to increase the rolldiameter at a given point, he allows the alloy to form, and conversely,in order to decrease the diameter of the roll at a given point heremoves the alloy with a scraping tool.

It will be recognized that great skill is required on the part of theoperator in order to properly control the size and shape of the exitrolls. Furthermore, even the best operator will often err when the widthor thickness of the material being coated is changed, For example, whenit becomes necessary to change width of strip being coated from 36inches to 48 inches, it is necessary for the operator to remove enoughiron-zinc alloy from the exit rolls on each side of the existing stripso as to make the entire roll face a true cylinder. Since he has nomeans of measuring the amount of material removed (or left on the exitroll) he often is forced to operate under conditions which do notproduce prime product. Likewise if the strip wanders off the center-lineof the exit rolls, one edge of the strip contacts roll surfaces whichare not conditioned in the same way as the rest of the roll surface,resulting in a non-uniform application of zinc. It is also known thatthe scraping operation removes the iron of the exit roll which hasalloyed with the zinc. Applicant has observed that the conventional exitroll in normal operation will lose on the order of A inch of itsdiameter in a week due to the constant alloying action and subsequentdressing and shaping of the roll. Depending upon the particular machine,the normal size of the exit rolls may be 6 to 12 inches in diameter.

In addition, the passage of the strip between a pair of exit rolls,particularly in the case where the rolls are driven at a speed differentfrom that of the strip passing between them, causes great amounts ofwear and removal of alloy at the areas of contact thereby resulting inroll surfaces which are no longer true cylinders. Finally, it iscustomary to form grooves in the surface of the exit rolls. Thesegrooves are helpful as is well known in the art for oxide control andcontrol of the weight of coating material applied to the strand. Likethe surface of the exit roll, these grooves tend to alloy and fill upprogressively, thereby changing the coating weight applied to thestrand. The operator, just as he scrapes the surface of the roll, willcustomarily clean out these grooves by means of a grooving tool. Onceagain he is depending entirely upon his own judgment, skill andexperience to determine the amount of grooving which is necessary inorder to apply a desired thickness or weight of coating material.

Pot or sinker rolls as well as stabilizer rolls operate completely belowthe surface of the molten zinc. Therefore, they cannot be repaired ormaintained unless the operation is stopped and they are raised from thebath. For this reason, the rolls should be as inert to the influences ofthe operation as possible. They must resist dimensional growth due toalloy formation as well as roughening or spalling which occurs when thealloy layer sloughs off, and wearing due to abrasion with the strip.

Since sinker rolls and stabilizer rolls are located under the bathsurface and cannot therefore be shaped by the operator, other materialssuch as Type 316 stainless steel have been used. Even this stainlessdissolves in zinc more rapidly than the material of this invention and,since it is a stable austenitic steel, it cannot be hardened to providethe wear resistance provided by the material of this invention.

SUMMARY OF THE INVENTION In its broadest aspects, the invention of thisapplication resides in the discovery that a particular steel alloy canbe used for exit rolls, stabilizer rolls and sinker rolls in a strandgalvanizing operation to achieve great economies and superior results.

It is a primary object of this invention to provide a coating method androlls for use therewith which are dimensionally stable and which do notrequire continual maintenance by the operator.

Another object of the invention is the provision of a coating method andexit rolls therefor which will permit the application of more uniformzinc coatings without continual shaping by the operator.

Another object of the invention is to provide a stabilizer or sinkerroll which will be dimensionally stable, will resist solution in moltenzinc and have adequate wear resistance so that it can operate withoutmaintenance for extended periods of time.

Still a further object of this invention is to provide rolls for use inmolten zinc which are chemically and dimensionally stable, and wettablebut resistant to the formation of an iron-zinc alloy.

Still a further object of the invention is the provision of an exit rollfor a galvanizing operation which has longer life between re-machiningoperations, and longer ultimate life.

It will be understood that the formation of an ironzinc alloy on therolls during the coating step, and the subsequent Scraping of this alloyfrom the exit rolls results in an increased dross formation, both in thebath and deposited on the strip, with the consequent reduction inquality and yield. It is therefore another object of this invention toreduce the formation of such dross on the surface of the coating bathand on the coated strip.

Still a further object of the invention is to provide a coating methodand exit roll therefor which, by eliminating the necessity for scrapingor shaping by the operator, permits substantially automatic control ofthe operation, thereby giving much greater control over the coatingweight and thickness. This is particularly important in view of theincreasing use of galvanized steels in products which previously havebeen made from uncoated steel, these new products often requiringimproved characteristics of the coated surface.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a photomicrograph of thecross-section of the surface layers of conventional exit roll materialsubmerged in a typical zinc galvanizing bath maintained at 850 F. forone hour.

FIG. 2 is a photomicrograph at the same magnification as FIG. 1 butshowing a sectional view of the surface layers of a sample rollaccording to this application subjected to the same conditions as theexit roll of FIG. 1.

FIG. 3 is a photomicrograph of a portion of FIG. 2 at a muchmagnification.

DESCRIPTION ON THE PREFERRED EMBODIMENT The invention relates to the useof a specific steel in three specific applications in stripgalvanizingexit rolls, stabilizer rolls, and sinker rolls. The sinkerroll or pot roll is a smooth idle roll submerged in the bath which holdsthe strip below the surface of the zinc. Stabilizer rolls are smoothidler rolls ordinarily used in coating lines using air knife or jetfinishing. One or more stabilizer rolls are positioned just below thesurface of the zinc bath as the strip is about to exit from the pot.Ordinarily there is one stabilizer roll located on the opposite side ofthe strip from the sinker roll. The stabilizer roll damps any vibrationin the strip and holds the strip fiat as it passes between the airknives so that the sharp gaseous jet will have a flat strip properlypositioned for Optimum finishing. Exit rolls are located at the surfaceof the bath to smooth and control the molten coating.

Briefly considered, the crux of this invention resides in the provisionof an exit roll, stabilizer or a sinker roll consisting essentially ofan alloy steel which is Wettable by molten zinc and yet is chemicallystable so as to severally limit the formation of an iron-zinc alloy andwhich is dimensionally stable with respect to the exposure to theordinary operating temperatures and hard enough to maintain its originalsize and shape in the presence of the abrasion of the strip passing overthe rolls.

More specifically, it has been found that some types of alloy steelspossess extremely high resistance to alloying with zinc and which arehighly resistant to warpage at the operating temperatures normallyencountered. Hardness of the alloy in the annealed condition isconsiderably greater than that of the conventional rolls, and the rollsof this invention can be further hardened so that the rate of wear dueto strip abrasion is very low. This combination of properties now makesit possible to produce and use rolls which are dimensionally stable,which are chemically stable, and which resist change in shape due todifferential wear.

The preferred rolls of this invention will consist essentially of anchromium-molybdenum-vanadium steel, ordinarily classified as an airhardening die steel.

Experimentation has indicated that an alloy steel having a compositionwithin the following range is entirely Within the above set forth range,the preferred analysis of the rolls of this invention will beapproximately 7.5% chromium, 1.0% molybdenum, 1.0% vanadium, 1.0%silicon, 0.40% carbon and 0.35% manganese. Manganese does not seem toadd must to the peculiar properties desired in this invention but anamount in this range is required in alloys of this type for knownpurposes.

As indicated above, it is desirable that the exit roll of this inventionhave dimensional stability. That is achieved by making the roll as hardas possible without causing susceptibility to cracking or excessivemachining cost. By way of example, exit rolls within the compositionrange set forth above and having a Brinell hardness in the range of200550 have been found entirely satisfactory. This may be compared withconventional exit rolls which have a Brinell hardness of about 100.

The rolls of this invention may be heat treated by austenitizing atabout 1900 F. and air cooling to produce a marte-nsitic structure.Before using hardened rolls they should be tempered at a minimumtemperature in excess of the coating temperature and up to approximately1200" F. This will minimize warpage and distortion in operation andreduce susceptibility to cracking. After heat treatment, the hardnesswill be Brinell 300 550.

Since the alloy described above is relatively expensive, it may bedesirable to produce the novel rolls according to this invention byalternate methods. This may be done by beginning with a mild steel rolland depositing the desired alloy on its surface by arc welding or anyother suitable process. The deposited surface layer should be built upto a thickness of from about inch to about /2 inch. In the case of exitrolls which are to be grooved according to conventional practice, thethickness of the surface layer should be near the upper end of the rangeset forth. The rolls may also be formed by centrifugal castingprocedures wherein a laminated roll may be cast with the desired alloyforming the surface.

The rolls of this invention have been tried in actual practice and havebeen found to be superior to known rolls. However, laboratory immersiontests were made in order to determine the alloying rate of the materialand to facilitate study of the undisturbed alloy layer. This wasnecessary to explain the unexpected good performance of the rolls ofthis invention.

FIGS. 1 through 3 show the results of a laboratory test to show thewettability and resistance to alloying of the rolls of this invention.Two test rolls, each three quarter inches in diameter by two inches longwere made up, one formed of the alloy described in this application andthe other of a conventional roll material with approximately 0.25%carbon. Both rolls were cleaned, prefluxed in zinc-ammonium chloride andimmersed for one hour in a zinc bath containing .12% aluminum andmaintained at 850 F.

In FIG. 1, a portion of the test roll is indicated at the referencenumeral 10, while the unalloyed or free zinc is the light material atthe top of the figure indicated at the reference numeral 12. Between theroll 10' and free zinc layer 12 is the iron-zinc alloy indicated at thereference numeral 14.

While not intending to be bound by theory, applicant believes from astudy of the photomicrograph of FIG. 1 that the conventional rollmaterial when subjected to immersion in a galvanizing bath forms a verythick,

hard, brittle alloy layer. The combination of hardness and thicknessapparently causes stresses resulting in the visible cracks of FIG. 1indicated at 16. This of course makes the alloy easily removed.Measurements of the alloy layer of the test roll shown in FIG. 1indicate that the thickness of the alloy is approximately .0098 inch.

In FIG. 2, the test roll formed of the alloy of this invention isindicated at 18. The free zinc layer, not clearly visible in FIG. 1 isindicated at 20 while the intermediate iron-zinc alloy is indicated at22.

The structure of this alloy and free zinc is more clearly shown in FIG.3. (FIGS. 1 and 2 are taken at a magnification of 200x, while FIG. 3 isa magnification of 1000 It will readily be observed that the alloy layeris extremely small in relation to the free zinc. Actual measurements ofthis test roll showed an alloy layer .0003 inch thick. This is athickness about that of the test roll of conventional material.

Additional test rolls similar to those shown in FIGS. 1 and 2 weremaintained in the zinc bath for a period of 24 hours. The test roll ofconventional material increased in size from .750 inch to .779 inch inone hour, and after twenty-four hours attained a diameter of .800 inch.By way of comparison, the test roll of the alloy of this inventionmaintained a relatively uniform size between .748 and .753 inch duringthe entire twenty-four hour period.

By way of brief summary, the rolls described above do not form aneasily-removed iron-zinc alloy as do conventional rolls and thereforeretain their shape, size and surface texture for a relatively longperiod of time. Consequently, the coating applied to the strip is moreuniform and free from dross or other surface defects. The greatlyincreased hardness reduces the rate of wear and in conjunction with thelower alloying rate allows the rolls to be used for substantially longerperiods of time before remachining, and also permits a much longerultimate roll life. These rolls lose ony about .020 inch in a weekscontinuous operation compared to a normal .250 inch. The elimination ofthe necessity for an operator to shape the exit roll during operationmakes it possible for these rolls to be used in conjunction with anautomatic coating weight sensing device, whereby substantially automaticcontrol of the operation may be obtained.

The operation of the coating line, including the steps of surfacepreparation, temperature or other conditions of the bath, do not form apart of the instant invention per se. As will be apparent fromconsidering the above description, this invention is concerned strictlywith the provision of improved rolls for a galvanizing operation and amethod of using these rolls.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a method of coating 21 ferrous strand with molten zinc includingthe steps of preparing the surface of the strand to insure receptivityto the molten zinc and passing said strand into a bath of molten zinc,the improvement which comprises the step of passing said strand throughsaid bath with at least one surface thereof in contact with an alloysteel roll, said roll having a Brinell hardness in the range of from 200to 550 said alloy steel being wettable by molten zinc and substantiallyresistant to the formation of an iron-zinc alloy and includingsubstantially 5 to 10% chromium, 0.5 to 2% molybdenum, 0.5 to 1.5%vanadium, 0.5 to 1.5% silicon, 0.20 to 0.60% carbon, and 0.20 to 0.80%manganese, the balance substantially iron.

2. The method of claim 1, wherein said alloy steel is heat treated to ahardness of Brinell 300550 by austenitizing at a temperature above 1900F., air cooling to produce a martensitic structure and tempering.

3. The method claimed in claim 1, wherein said alloy steel includessubstantially 7.5% chromium, 1% molybdenum, 1% vanadium, 1% silicon,0.40% carbon, and 0.35% manganese.

References Cited UNITED STATES PATENTS 1,695,916 12/1928 Comstockl26(E)X 2,069,260 2/1937 Merten 75126(E)X 2,740,729 4/1956 Hodil117102(M) 2,781,259 2/1957 Roberts 75126(E) 2,914,419 11/1959 Oganowski117114(A)X 2,952,568 9/1960 Diehl et a1. 117-114(A)X 3,163,525 12/1964Fletcher et al 75--126(E) ALFRED L. LEAVITT, Primary Examiner J. R.BATTEN, JR., Assistant Examiner US. Cl. X.R.

